1. Field of the Invention
The present invention relates to methods for producing stirred type fermented milk enriched with a whey protein. The present invention also relates to stirred type fermented milk enriched with a whey protein prepared by such a process.
2. Discussion of the Background
With respect to the quality of fermented milk such as yogurt, little water separation and high viscosity are regarded to be important as factors to determine the appearance and the texture of fermented milk. In order to improve these factors, a method of condensing raw material milk, a method of adding powdered skim milk, a method of adding a whey protein obtained by separation and condensation of whey produced from cow milk in a production of cheese, casein or the like, a method of adding a thickening polysaccharide or a method of adding a transglutaminase or the like is often used.
In particular, the addition of a milk whey protein and the use of a transglutaminase can prevent water separation and increase the viscosity without significantly changing the original flavor and texture of fermented milk.
Incidentally, fermented milk (yogurt) is roughly classified into a stirred type fermented milk (so-called early-fermentation yogurt) and a set type fermented milk (so-called late-fermentation yogurt) in accordance with its production method. In the case where a whey protein is used for a stirred type fermented milk, as shown in Japanese Patent Application Publication No. JP-A-7-104, the resulting yogurt has a coarse-grained structure, therefore, there has been a disadvantage that the resulting yogurt is likely to have a non-smooth texture.
Such a tendency becomes prominent as the addition amount of a whey protein is increased or the storage period of the resulting yogurt is longer. Therefore, even if it is intended to obtain a high effect by using a whey protein, it is difficult to increase the addition amount of a whey protein. Accordingly, there has been a limit on the improvement of the quality of stirred type fermented milk by using a milk whey protein.
In the case where a transglutaminase is used for fermented milk, as disclosed in Japanese Patent Application Publication No. JP-A-6-197688, a case in which a raw material milk is treated with a transglutaminase in advance prior to a fermentation step is common. In this case, it is necessary to retain the raw material milk and to subject the raw material milk to an enzymatic reaction, therefore, a large tank with a capacity equal to or greater than the production amount of the fermented milk is occupied during the enzymatic reaction time, and also an additional cost is required for an energy for maintaining a temperature necessary for the enzymatic reaction, and thus, there has been a disadvantage that the equipment use efficiency and energy efficiency are decreased. Further, a transglutaminase is still an expensive material, therefore, in comparison with other materials, a price advantage is not necessarily high.
In the production of a stirred type fermented milk, an attempt was also made in which a whey protein and a transglutaminase were added to a raw material milk and the resulting mixture was subjected to an enzymatic reaction thereby trying to obtain an synergistic improvement effect of both substances, however, the effect remained at an additive level, and an economical advantage was not observed.
Japanese Patent Application Publication No. JP-A-2000-4786 discloses a method comprising allowing a transglutaminase to act on a solution of a milk whey protein, and thereafter mixing the resulting reaction mixture in a raw material milk. According to this method, a yogurt, which has a smooth texture and goes down smoothly when swallowing, can be obtained. However, what is disclosed in the document is only a so-called late-fermentation yogurt obtained without crushing gel by stirring or the like, and the method does not apply to a stirred type yogurt, which is an object of the present invention.